I am conflicted....
As you can see here:
Yellow fires at a much higher rate of speed, though his aim is a bit low.
Looking at Mr. Green, shown here:
you can clearly see that he has a head shot. Now i know they are both made entirly of heads, but his is more of a head shot. Yellow's is more of a chest shot. Taking into consideration what curse said about the rail gun, I think that even though Mr. Green's gun appears to fire slow bullets that is just in bullet time and in reality Yellow's head would be pierced before he could fire a single shot.
Mr. Green PWNZ J00